BIA, or Bioelectrical Impedance Analysis, is a non-invasive measurement of body fat, lean muscle mass and hydration.
A small current is used to measure the resistance at difference frequencies against the various tissues in the body eg fat/lipid has a high resistance to the flow of current, therefore shows a high impedance reading.
Lean muscle and water are good conductors of current, therefore will show low resistance or impedance readings.
The impedance readings are then translated using validated algorithms into estimations of body fat, lean muscle and water (including water inside and outside the cells when using the higher frequencies). The algorithms used for each BIA device (or brand) is likely to be different, which is why you might have different results using different machines. The accuracy will depend on the validation of the equation used and how it compares with DXA and other gold standards. This is always something to take into account when choosing a BIA device.
The accuracy of the measurement also depends on skin resistance. When we are using a light current to measure inside the body, the thickness of the skin and the connection to the electrode is of utmost importance. Understanding this, think of the thickness of the soles of your feet and palms of your hand in comparison to the thin skin on the tops of your hands and feet. This is why Bodystat use their calibrated, medically graded electrodes on top of the hands and feet, rather than stand-on scales. See: bodystat_elctrodes_vs_scales
Watch this explainer video to understand in more detail how this technology works.